JP3242566B2 - Method for preparing analytical sample, method for analyzing impurities, method for preparing high-purity phosphoric acid, and method for manufacturing semiconductor device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to the use of phosphoric acid,
More specifically, a method for preparing an analytical sample for quantitatively analyzing radioactive impurities contained in phosphoric acid, a method for analyzing an impurity for quantitatively analyzing radioactive impurities contained in phosphoric acid, and a method for substantially analyzing radioactive impurities. The present invention relates to a method for preparing high-purity phosphoric acid which is not contained in general and a method for manufacturing a semiconductor device using high-purity phosphoric acid as a processing solution. The present invention can be advantageously used particularly in a processing step in which phosphoric acid is used as a processing liquid in a semiconductor device manufacturing process.

[0002]

2. Description of the Related Art As is well known, phosphoric acid is frequently used as an etching agent in the process of manufacturing semiconductor devices such as ICs and LSIs. That is, for example, wet etching using phosphoric acid as a processing solution is used to selectively remove an insulating film formed on a semiconductor substrate, for example, a thin film of silicon nitride. Phosphoric acid, however, contains the radionuclide ²¹⁰ Po (T _1/2 = 138.4d) as an impurity, so during the phosphoric acid treatment this ²¹⁰ Po is adsorbed on the silicon substrate or deposited by electroless plating. It is possible to be. After a series of semiconductor device manufacturing processes are completed, ²¹⁰ Po previously adsorbed on the silicon substrate emits α-rays, which can cause soft errors (memory errors) in electronic devices such as semiconductor memories. is there. The reason that ²¹⁰ Po is contained in phosphoric acid is mainly due to the composition of the phosphate rock used as a starting material in the preparation process. That is, as described in, for example, Keiichiro Kubo, "Inorganic Industrial Chemistry", pp. 121-129, Asakura Shoten, published on October 5, 1967, the wet method [phosphoric acid is decomposed by sulfuric acid To obtain phosphoric acid) or a dry process [reducing phosphorous ore to obtain phosphorus, further oxidizing phosphorus to phosphorus pentoxide, and dissolving this phosphorus pentoxide in water to obtain phosphoric acid]. Although possible, the radioactive impurities contained in the raw rock ore can remain in the generated phosphoric acid without being removed in the process of producing phosphoric acid.

The occurrence of soft errors in electronic devices is not limited to ²¹⁰ Po described above.
According to the findings of the present inventors, other radioactive impurities contained in phosphoric acid can also be involved. The impurities expected to be involved are, for example, Bi (214, 210, 215, 21).
2), Pb (214, 210, 211, 212), Ac
(227,228), Th (234,230,231,
227, 232, 228). In particular, Pb, B
The involvement of i and Po is important, and it is therefore desirable to eliminate the adverse effects of such radioactive impurities.

[0004] The occurrence of soft errors in electronic devices as described above will become more apparent, especially with an understanding of the background art as described below. For example,
Japanese Patent Application Laid-Open No. Hei 3-207596 discloses the existence of a small amount of radioisotopes, ²¹⁰ Pb and ²¹⁰ Po, contained in a solder material formed as a Pb alloy as a cause of memory errors in semiconductor devices such as ICs and LSIs. Thus, the content of such radioisotopes was less than 5 ppb and the count of radioactive α particles was 0.09 CPH / cm ^2.
It teaches that: In the lower part of page 2 of this publication, ²¹⁰ Pb and ²¹⁰ Po are radiation decay as follows: ²¹⁰ Pb → (β decay) → ²¹⁰ Bi → (β decay) → ²¹⁰ P
o → (α decay) → ²⁰⁶ Pb emits β-rays and α-rays as a result, but above all ²¹⁰
It is disclosed that α decay when Po is converted to ²⁰⁶ Pb causes a memory error.

Further, Japanese Patent Publication No. 60-15152 discloses that
To provide a highly integrated resin-encapsulated semiconductor memory device that does not cause a soft error due to α-rays, and includes a resin element having a total content of uranium and thorium of 0.2 ppb or less, Α having a thickness of 40 μm
It teaches providing a line shielding layer. By the way, as is well known, polonium ²¹⁰ Po is described in, for example, Chemical Dictionary 8, pages 811 to 812, Kyoritsu Shuppan, February 1962.
138.401-day (d) half-life radioactive isotope discovered with radium from pitch blends by the Curie and his wife as described in US Pat. Usually, the determination of ²¹⁰ Po is carried out by measuring the α-activity released from it, and for separation from other elements, 1) precipitation method, 2) electrodeposition method, 3) volatilization method, 4) Ion exchange resin method, 5) Solvent extraction method and the like are used, but the method of electrodeposition on a metal plate and then volatilization in vacuum is most reliable.

In the prior art, α existing in a liquid
As a method for analyzing a radiation emitting nuclide, for example, Japanese Patent Application Laid-Open
There is a method for analyzing α-ray emitting nuclides present in a liquid such as wastewater discharged from nuclear facilities to the environment, which is disclosed in Japanese Patent No. 8645. According to the analysis method described in this publication, after a cathode electrodeposition plate serving also as a bottom plate is attached to an electrodeposition cell made of a resin material such as tetrafluoroethylene resin, a sample liquid and an electrolyte solution are injected into the electrodeposition cell. Then, the anode is immersed in the liquid mixture, and the lower part of the electrodeposition cell is inserted into a temperature-controlled medium to control the temperature of the liquid mixture and to conduct electricity between the electrodes. After the α-ray emitting nuclide in the sample liquid is electrodeposited on the electrodeposited plate, the electrodeposited plate can be subjected to an α-ray measuring device such as an α-ray height analyzer to analyze the nuclide. You can also improve the accuracy of your analysis,
In order to perform the correction, it is also possible to add a known amount of a different α-ray nuclide in advance and then perform the analysis. However, the analysis method described in this publication discloses that ²¹⁰ Po
Nothing is known about the undesired effects of the radioisotope, etc., nor is the analysis of the radioisotope discussed.

Further, many related techniques are known from US patents, such as a method for detecting radioisotopes, a method for recovering an actinide element (uranium and the like) from a phosphoric acid waste solution by a wet method, and a method for regenerating a phosphoric acid waste solution. For example, US Pat. No. 4,336,45
No. 1 is that uranium series and thorium series radioactive isotopes are eluted from a leaching solution (acids such as nitric acid and hydrochloric acid) on a metal plate, or the isotope is transferred by electroless plating or electrolytic plating. It is disclosed to detect radiation decay thereafter. However, there is no mention in this patent of the analysis of polonium in phosphoric acid. U.S. Pat. No. 3,983,219 discloses in the description of the prior art that ²¹⁰ Po could be deposited on a platinum cathode from a 4-8N nitric acid solution by an electrolytic method. are doing.

Further, with respect to the method for purifying phosphoric acid, for example, US Pat. Nos. 4,450,142, 4,162,230, 4,200,620, and 5,200 No. 316,748. Regarding the method for regenerating the phosphoric acid waste liquid, for example, US Pat. Nos. 4,749,455 and 4,615,455
No. 776.

[0009]

As will be understood from the above description, in order to quantitatively analyze impurities in phosphoric acid, particularly radioactive impurities, to a trace amount region, the impurities are quantitatively separated from phosphoric acid as a matrix. In order to improve the quantitative accuracy, it is necessary to prepare a thin-film measurement sample. In addition, it is necessary to provide a process for removing radioactive impurities in phosphoric acid using an analytical method, and to control the process.

Further, in the manufacture of a semiconductor device, in order to reduce soft errors caused by radioactive impurities in phosphoric acid, use of phosphoric acid having a low concentration of radioactive impurities such as ²¹⁰ Po is required. It is necessary to manufacture a semiconductor device while preventing the attachment of impurities. Accordingly, a first object of the present invention is to prepare an analysis sample for quantitatively analyzing radioactive impurities contained in phosphoric acid with respect to phosphoric acid which has not been controlled in the prior art. It is to provide a way to do it.

A second object of the present invention is to provide a method for quantitatively and highly accurately analyzing radioactive impurities contained in phosphoric acid in a trace amount region. A third object of the present invention is to provide a method for purifying phosphoric acid containing impurities to prepare high-purity phosphoric acid that can be advantageously used in the manufacture of semiconductor devices and the like.

A fourth object of the present invention is to provide a method for manufacturing a semiconductor device by processing a substrate to be processed, which does not cause a soft error failure during the manufacturing process. is there. The above and other objects of the present invention will be easily understood from the following detailed description.

[0013]

According to the present invention, the first object of the present invention is to prepare an analytical sample for quantitatively analyzing radioactive impurities contained in phosphoric acid. This can be achieved by a method for preparing an analysis sample, which is characterized in that only impurities are precipitated and separated.

A second object of the present invention is to quantitatively analyze radioactive impurities contained in phosphoric acid, by precipitating only impurities to be analyzed from the phosphoric acid, separating them, and separating the obtained separated product. This can be achieved by a method for analyzing impurities in phosphoric acid, which is characterized in that it is subjected to analysis as a sample. A third object of the present invention is to purify phosphoric acid containing impurities to prepare high-purity phosphoric acid, wherein only impurities are precipitated from phosphoric acid to be purified and the impurities are separated. This can be achieved by a method for preparing phosphoric acid.

A fourth object of the present invention is to manufacture a semiconductor device by processing a substrate to be processed, wherein the content of impurities contained in at least one processing step included in the manufacturing process is Pb, Defined by the concentration of radioisotope selected from the group consisting of Bi and Po,
The method can be achieved by a method for manufacturing a semiconductor device, wherein phosphoric acid having a concentration of 10 ⁻³ Bq / ml or less is used as a treatment liquid. In order to achieve these objects, the step of precipitating only radioactive impurities from phosphoric acid and separating the radioactive impurities comprises heating the phosphoric acid, immersing the semiconductor substrate in the heated phosphoric acid, and placing the semiconductor substrate on the substrate. It can be carried out by precipitating only the impurities or by changing the phosphoric acid so as to exhibit weak acidity, and then precipitating only the impurities on the electrode by electrolytic deposition of the weakly acidic phosphoric acid.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION
Prior to the description, the purity of phosphoric acid used as an etchant in the manufacture of a semiconductor device such as a semiconductor memory will be described, and in addition to this description, a comparison will be made. Because hydrochloric acid,
The purity of nitric acid, hydrofluoric acid, and aqueous ammonia is also described. Phosphoric acid as an etching agent is usually used in the form of a stock solution, that is, at a concentration of 85% by weight.

The control of impurities in phosphoric acid is usually performed by a phosphate reagent manufacturer according to the impurity control items and control levels (contents, ppm) as shown in the following table. Impurity Item Chloride Sulfate Nitrate Ba Ca Cu Fe Control Level (ppm) 0.1 20 0.1 0.1 0.1 0.1 0.1 Impurity Item K Li Na Ni Sr Mn As Management Level (ppm ) 1 0.1 0.2 0.1 0.1 0.1 0.1 Also, for the purpose of comparison, analysis test results show that hydrochloric acid (20% aqueous solution), which is also used as an etching agent,
Nitric acid (68% aqueous solution), hydrofluoric acid (38% aqueous solution),
The purity of the aqueous ammonia (20% aqueous solution) is as described in the following table. Impurity content (ppb) Item Hydrochloric acid nitric acid hydrofluoric acid ammonia water Al 0.01 0.02 0.03 0.02 Sb <0.02 <0.02 <0.05 <0.01 As <0. 02 <0.001 Be <0.002 <0.002 <0.005 <0.005 Bi <0.01 <0.01 <0.02 <0.01 Cd <0.005 <0.001 <0 0.003 <0.001 Ca <0.03 0.04 0.005 0.03 Cr <0.005 <0.01 <0.005 <0.01 Co <0.005 <0.02 <0.05 <0.01 Cu 0.013 <0.01 0.02 0.03 Au <0.002 <0.002 <0.005 <0.005 Fe 0.045 0.02 0.02 0.03 Pb < 0.005 <0.01 <0.01 <0.01 Li 0.01 <0.01 <0.002 <0.005 Mg 0.025 0.04 0.01 <0.005 Mn <0.01 <0.005 <0.01 <0.005 Ni <0. 02 <0.01 <0.03 <0.03 K <0.01 <0.005 <0.005 <0.005 Ag <0.001 <0.001 <0.002 <0.001 Na <0 .01 0.03 0.02 0.04 Sr <0.02 <0.02 <0.05 <0.01 Sn <0.01 <0.02 <0.05 <0.01 Zn <0.005 0.03 0.017 0.04 Th ^* <0.001 <0.001 <0.001 <0.001 U ^* <0.001 <0.001 <0.001 <0.001 Here, the impurity Th ^{* And} U ^* are measured according to ICP (Inductively Coupled Plasma). Other impurities are measured with a flameless atomic absorption spectrophotometer. As can be seen from the above table, the impurities contained in phosphoric acid are 21.31 ppm. In comparison, the hydrochloric acid impurity shown as a comparison is <0.275 ppb, the nitric acid impurity is <0.358 ppb, the hydrofluoric acid impurity is <0.421 ppb, and the ammonia water impurity is Is <0.325 ppb. That is, although the impurity control level of phosphoric acid is on the order of ppm, hydrochloric acid,
The control levels of impurities such as nitric acid, hydrofluoric acid and aqueous ammonia are on the order of ppb. Phosphoric acid can therefore be said to be less pure compared to other conventional etchants.

Also, the impurity control level of sulfuric acid is similarly
It is clear from the literature that this is on the order of ppb (Do
nald Potter et al., "Ultra Trace Analysis of Semi
conductor Grade Reagents by ICP-MS ", ANALYTICAL S
See CIENCES VOL. 7, SUPPLEMENT1991, pp 467-470; the total amount of impurities in sulfuric acid is 0.902 ng / g =
0.902 ppb). In other words, it is clear that the analysis of impurities in phosphoric acid is more difficult than in sulfuric acid, and that it is not easy to purify phosphoric acid. That is, the test standard of phosphoric acid as an etching agent is also mild compared with other common etching agents such as hydrochloric acid, nitric acid, hydrofluoric acid, sulfuric acid, and ammonia water. According to the understanding of the present inventors, such difficulties in the analysis and the high purification are due to the physical and chemical properties that phosphoric acid is non-volatile and has a high viscosity, in addition to the characteristics of the industrial production method of phosphoric acid. [Phosphoric acid is capable of boiling at 158 ° C., measured in an 85% by weight aqueous solution,
The kinematic viscosity at 20 ° C. is 28 centistokes].

The present invention surprisingly overcomes the difficulty of analyzing impurities in phosphoric acid and the difficulty of purifying phosphoric acid, which could not be overcome by conventional techniques. Is what you can do. In one aspect of the present invention, there is provided a method for preparing an analytical sample for quantitatively analyzing radioactive impurities contained in phosphoric acid, comprising separating only an analyte from the phosphoric acid and separating the same. It is characterized by. The analysis sample is preferably in the form of a thin-film disc in consideration of its handleability, compatibility with an analyzer, and the like, but may be in another form as necessary.

The preparation of such an analysis sample can be embodied in various modes. In a preferred embodiment, after phosphoric acid is heated, a semiconductor substrate, for example, a tangible material made of silicon (Si), gallium arsenide (GaAs), or the like is immersed in the heated phosphoric acid to precipitate impurities to be analyzed on the substrate. Including that. When heating phosphoric acid, a preferable heating temperature is usually about 120 to 170 ° C, more preferably about 140 to 160 ° C. As another method of this method, if necessary, instead of the semiconductor substrate, silicon or a compound thereof, for example, silicon carbide (SiC) or the like is immersed in phosphoric acid to adsorb the impurities to the silicon or the compound. Is also good. Here, silicon or a compound thereof can be used in various forms, for example, in the form of particles, powder, pellets, and the like. Furthermore, the heating of the phosphoric acid may be performed, if necessary, after the semiconductor substrate is immersed in the phosphoric acid. Another preferred embodiment includes, after changing the phosphoric acid to exhibit a weak acidity, depositing an impurity to be analyzed on the electrode by electrolytic deposition or electrodeposition of the weakly acidic phosphoric acid. . Where "weakly acidic"
Means, as can be said throughout the present specification, that the acidity of the phosphoric acid has been moderated by adding an appropriate amount of alkali to a level suitable for the desired use conditions of the phosphoric acid. I do. A suitable weakly acidic range is therefore any level below pH = 7, preferably about 2 to about 5. Further, the electrolytic deposition method or the electrodeposition method can be performed according to a known method.

In the method for preparing an analytical sample according to the present invention, the impurities to be analyzed in the phosphoric acid are, in particular, Pb, B
a radioisotope selected from the group consisting of i and Po, especially ²¹⁰ Po, ²¹⁰ Pb or ²¹⁰ Bi. However, the method of the present invention is method for analyzing impurities to further illustrate the preparation of the analytical sample and below, including the production method of the preparation method and a semiconductor device of high purity phosphoric acid, ²¹⁰ Po, ²¹⁰
Radioactive impurities other than Pb or ²¹⁰ Bi, such as Ac, T
h, U, etc., can be advantageously applied.

According to another aspect of the present invention, there is provided a method for quantitatively analyzing radioactive impurities contained in phosphoric acid. The method comprises the step of precipitating and separating only impurities to be analyzed from the impurity-containing phosphoric acid, and It is characterized in that the obtained isolate is subjected to analysis as a sample. Here, the step of precipitating only the impurities to be analyzed from the phosphoric acid containing impurities can be performed according to the impurity precipitation or adsorption step used in the preparation of the analysis sample described above.

In one preferred embodiment of the analysis method of the present invention, after the phosphoric acid containing impurities is heated, a semiconductor substrate such as silicon (Si) or gallium arsenide (GaAs) is immersed in the heated phosphoric acid. Depositing an impurity to be analyzed thereon and measuring the intensity of α-rays, β-rays or γ-rays emitted from the precipitate. In another preferred embodiment, the analysis method of the present invention comprises, after changing the impurity-containing phosphoric acid to exhibit a weak acidity,
An impurity to be analyzed is deposited on the electrode by the electrolytic deposition method of the weakly acidic phosphoric acid, and α rays, β emitted from the deposit are deposited.
Measuring the intensity of the gamma or gamma rays.

In this analysis method, the impurity to be analyzed is preferably a radioisotope selected from the group consisting of Pb, Bi and Po, particularly ²¹⁰ Po, ²¹⁰ Pb or
²¹⁰ Bi. For example, the handling as radioisotopes ²¹⁰ tracer ²¹⁰ Po for quantitative analysis of identity position body by adding isotopes ²¹⁰ Po when a Po as an object to be analyzed impurity, the radioisotope in ²¹⁰ Pb In some cases, isotopes of ²¹⁰ Pb were added to convert the isotope to ²¹⁰ Pb.
It is preferable to handle it as a tracer for quantitative analysis, or to add a ²¹⁰ Bi isotope when the radioisotope is ²¹⁰ Bi and handle the isotope as a tracer for ²¹⁰ Bi quantitative analysis. In the method of the present invention, preferably, the impurity can be analyzed by analyzing a daughter nuclide or a progeny of the radioisotope as the impurity to be analyzed.

In another aspect, the present invention is a method for preparing phosphoric acid containing impurities by purifying phosphoric acid containing impurities, wherein only impurities are precipitated from phosphoric acid to be purified and separated. It is characterized by doing. The method for preparing high-purity phosphoric acid according to the present invention includes various preferred embodiments. In one preferred embodiment, the method of the present invention comprises, after heating the phosphoric acid to be purified, immersing the semiconductor substrate in the heated phosphoric acid to precipitate the impurities on the substrate, and removing the semiconductor substrate from the heated phosphoric acid. It is characterized by being removed. As another method of this method, if necessary, instead of the semiconductor substrate, silicon or a compound thereof may be immersed in phosphoric acid to adsorb the impurities to the silicon or the compound. Silicon or a compound thereof is described above.

In another aspect of the method of the present invention, the impurity to be purified is deposited on an electrode by an electrolytic deposition method of the weakly acidic phosphoric acid after the purified phosphoric acid is changed to exhibit weak acidity. And removing the electrode from the weakly acidic phosphoric acid. Further, in another aspect of the present invention, there is provided a method for manufacturing a semiconductor device by processing a substrate to be processed, wherein the content of impurities contained in at least one processing step included in the manufacturing process is reduced. , Pb, Bi and Po, defined by the concentration of radioisotope selected from the group consisting of 10
It is characterized in that phosphoric acid of ^-3 Bq / ml or less is used as a treatment liquid. In the method for manufacturing a semiconductor device of the present invention, steps other than the processing step using phosphoric acid as a processing solution can be common steps in this technical field, and can be arbitrarily combined. Preferably, the phosphoric acid is
It is used as an etchant for selectively removing an arbitrary thin film on a substrate to be processed.

The content of impurities contained in phosphoric acid is
It is preferably as small as possible, so that Pb, Bi and
Concentration of radioisotope selected from the group consisting of
Defined in degrees, 10^-FourYes, it is at a level of Bq / ml or less
It is profitable. In the method for manufacturing a semiconductor device according to the present invention,
As small as possible in phosphoric acid as a processing solution.
The impurities to be rare are preferably²¹⁰Po,²¹⁰Pb
Or ²¹⁰Bi. Phosphoric acid as a treatment liquid is preferable.
More preferably, the phosphoric acid that is to be used as a processing solution is
After heating, the semiconductor substrate is immersed in the heated phosphoric acid to
Depositing the impurities on a substrate of
Prepared by removing the body from the heated phosphoric acid
Phosphoric acid, which is planned to be used as a processing solution,
Electrolysis of the weakly acidic phosphoric acid
Depositing the impurities on the electrodes by a deposition method, and
Prepared by removing the electrode from the weakly acidic phosphoric acid
Spent or otherwise planned to be used as processing solution
Silicon or its compound to the phosphoric acid
Adsorbs and removes the impurities on the element or its compound
It was prepared by These phosphoric acids
Is described above.

In the method of manufacturing a semiconductor device according to the present invention, preferably, the treatment using the phosphoric acid as a treatment liquid is performed by a method of forming a protective film in a non-phosphoric acid treated region (unused region other than desired) of the substrate to be treated. Can be implemented in existence. Here, the protective film may be already formed on the substrate to be processed, or may be newly formed before the phosphoric acid treatment. Such a protective film is preferably made of a nitride (Si ₃ N ₄ ) or an oxide.
It is a thin film of (SiO ₂ ).

In the method of manufacturing a semiconductor device according to the present invention, the processing using the phosphoric acid as a processing solution can be employed in various stages or steps of the manufacturing process. As an example of a preferred process, for example, L
Selection of OCOS structure Mask for SiO ₂ film formation, gate material,
Silicon nitride used as a passivation film, etc.
This is a wet etching of the (Si ₃ N ₄ ) film. Further, as a method of wet etching, immersion etching, spray etching, jet etching, steam etching, reduced pressure etching, and the like generally used in this technical field can be arbitrarily used.

The phosphoric acid etching process according to the present invention comprises:
Although not limited to those described below with reference to the drawings, for example, as shown in FIG.
The present invention can be applied to etching of a silicon nitride (Si ₃ N ₄ ) film as a mask for forming a silicon oxide film having a LOCOS structure. First, as shown in FIG. 1A, a silicon oxide film (SiO ₂ film) 12 and a silicon nitride (Si ₃ N ₄ ) film 13 are sequentially formed on a silicon substrate 11 by using a conventional lithography technique. . Next, only a desired region where a silicon oxide film is to be grown by the LOCOS method is selectively removed by a conventional lithography technique. As a result, FIG.
As shown in FIG. 7, the composite film of the silicon nitride film 13 and the silicon oxide film 12 is removed only in a desired region 14 on the silicon substrate 11. Subsequently, as shown in FIG. 1C, in the region 14 formed in the previous selective removal process,
A silicon oxide film (SiO ₂ film) 1 on the silicon substrate 11
Grow 5 This can be done by oxidizing the underlying silicon substrate 11 by a conventional LOCOS method (the patterned silicon nitride film 13 can act as a mask against oxidation). After the formation of the LOCOS oxide film, the silicon nitride film 1 previously used as a mask
3 is selectively etched with phosphoric acid. The phosphoric acid used here is high-purity phosphoric acid obtained by the present invention.
An oxide film having a cross-sectional structure as shown in FIG.

The present invention encompasses a variety of methods, as described above, including the separation or precipitation of radioactive impurities from phosphoric acid. Separation or precipitation of impurities, which is one of the methods of the present invention, involves weakening a phosphoric acid sample and then depositing the impurities on an electrode (cathode) by an electrolytic deposition method or an electrodeposition method. Specifically, the precipitation of impurities by the electrolytic deposition method and the subsequent quantitative analysis of the impurities can be advantageously performed as follows.

Addition of tracer A tracer is added to a phosphoric acid aqueous solution as an analysis sample. For example, if the impurity to be deposited on the electrode is ²¹⁰ P
If it is o, the isotope of ²¹⁰ Po is added and the isotope is treated as a tracer for quantitative analysis of ²¹⁰ Po. Adjustment of pH After adding the pH indicator, an appropriate amount of alkali is added to make the analysis sample weakly acidic.

Precipitation of Impurities A sample solution is injected into an electrodeposition cell, and impurities are deposited on an electrode (cathode plate) by an electrodeposition method. A thin film impurity is formed. Quantitative Analysis of Impurities From the measurement of the radioactivity of the electrodes, the radioactivity released from the impurities and the tracer is determined, and the impurities are quantified based on the radioactivity of the tracer.

According to the above-mentioned method, it is possible to separate impurities from the phosphoric acid matrix and to prepare a thin film-shaped measurement sample. It is possible to quantitatively analyze up to.

[0035]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to some embodiments. It should be understood that the present invention is not limited to the embodiments described below. Example 1 Separation and Quantitative Analysis of ²¹⁰ Po Eleven types of wet etching phosphoric acid solutions (all 85 wt% aqueous solutions) commercially available from three phosphoric acid manufacturers (hereinafter referred to as A, B and C for convenience) were used as sample solutions. Prepared as. Tracer ²⁰⁹ was added to each of 5 mL of these sample solutions ^.
1 mL of a hydrochloric acid standard solution of Po was added. The tracer solution used here was from Oak Ridge National Laboratory, and the T _1/2 of ²⁰⁹ Po was 103y. Then, after adding cresol red as a pH indicator,
Add an appropriate amount of aqueous sodium hydroxide solution to adjust the pH of the sample solution.
Was adjusted to about 2.

Subsequently, the sample solution after the pH adjustment was injected into the electrodeposition cell. The electrodeposition cell used here is schematically shown in FIG. 2 and includes a main body 1 made of Teflon (trade name of DuPont, polytetrafluoroethylene) resin, an anode 3 made of platinum, and a cathode made of stainless steel plate. 2 and an electrolytic solution 4 in the cell body 1. A power supply 5 was connected to the anode 3 and the cathode 2. The cell capacity was 20 mL, and the distance between the electrodes was about 1 cm. Using such an electrodeposition cell, electrodeposition was performed under the following conditions, and polonium (Po) contained in the sample solution was deposited on the surface of the stainless steel electrode plate 2.

Constant current mode: current = 0.16 A / cm ² , voltage = 5
10 V Temperature: room temperature Electrodeposition time: 3 hours After completion of the electrodeposition treatment, the stainless steel electrode plate was taken out of the electrodeposition cell and dried. Next, regarding polonium deposited in the form of a thin film on the surface of the electrode plate, ²⁰⁹ Po (4.877) was used.
(MeV) and ²¹⁰ Po (5.305 MeV). For the measurement of this α-ray, a commercially available tracer measuring device, which is connected to a low background type α-ray detector (sensible area 450 mm ² ) manufactured by Seiko EG & G, is connected to an α-ray measurement system manufactured by NAIG. did.
Next, based on the result of the α-ray measurement, ²¹⁰ Po was quantified from the count number of ²⁰⁹ Po and the recovery rate. The results obtained are shown in Table 1 below. Table 1 Variation of ²¹⁰ Po Concentration by Phosphoric Acid Maker, Grade, and Lot Phosphoric Acid Maker Grade Lot No. ²¹⁰ Po Concentration (X10 ^-3 Bq / mL) AI 1 1.16 ± 0.06 AI 2 <0.01 A II 3 0.74 ± 0.09 A II 4 0.02 ± 0.005 A III 5 0.33 ± 0.03 A III 6 0.01 ± 0.005 B IV 7 0.92 ± 0.09 B IV 8 <0.01 B IV 9 <0.01 C V1 0.16 ± 0.02 CV 2 1.14 ± 0.09 From these measurement results, the polonium contained in the sample solution was determined with high accuracy. In addition to the quantitative analysis, it was found that 80% or more of the polonium was deposited on the surface of the stainless steel electrode plate and could be recovered. In other words, a high-purity phosphoric acid solution substantially free of polonium, which is a radioactive impurity, could be prepared from a commercially available phosphoric acid solution. Further, in the phosphoric acid sample solution used in this example, apart from the above-mentioned two nuclides ²⁰⁹ Po and ²¹⁰ Po, like the polonium, they are deposited on the surface of the stainless steel electrode plate and obstruct the measurement of those nuclides. No nuclides were found. Furthermore, from the above results, the ²¹⁰ Po concentration is
It has also been found that the variation can be greater for different lots than for different phosphoric acid manufacturers and phosphoric acid grades. Example 2 Separation and quantitative analysis of ²¹⁰ Po A phosphoric acid solution (85% by weight aqueous solution) for wet etching from various phosphoric acid manufacturers was prepared as a sample solution. The concentration is known (1.0 × 10 ⁻² Bq /
1 mL of Tracer ²⁰⁹ Po hydrochloric acid acid standard solution. The tracer solution used here was manufactured by Oak Ridge National Laboratory as in Example 1 above.
T _1/2 of ²⁰⁹ Po was 103y. After the sample solution after the addition of the tracer solution was left overnight, methyl red was added as a pH indicator, and an appropriate amount of an aqueous sodium hydroxide solution was added dropwise to make the solution weakly acidic (pH = about 2).

Subsequently, the sample solution after the pH adjustment was injected into the same electrodeposition cell (see FIG. 2) as that used in Example 1 above. Electrodeposition was performed under the same conditions as those described in Example 1 above, and polonium (Po) contained in the sample solution was deposited on the surface of the stainless steel electrode plate. After energization for a predetermined time was completed, ammonia water was added dropwise to make basic the polonium deposited on the surface of the electrode plate, in order to prevent the polonium from dissolving again in the sample solution.

After the completion of the electrodeposition treatment, the stainless steel electrode plate was taken out of the electrodeposition cell and dried. Using this electrode plate as a radiation measurement sample, ²⁰⁹ P was measured by α-ray spectrometry.
o and ²¹⁰ Po peaks were measured. FIG. 3 shows an example of the obtained measurement results. Next, based on the measurement results shown in FIG. 3, the following formula: A = BC / D (where A is the amount of ²¹⁰ Po in the sample solution, B is the amount of ²⁰⁹ Po added, and C is the amount of ²⁰⁹ Po Count number, and D is ²¹⁰ P
The amount of ²¹⁰ Po was quantified according to the following formula: From the results of ²¹⁰ Po quantification in each sample solution, it was found that ²¹⁰ Po can be quantitatively analyzed with high accuracy and high reliability up to a level of 10 ⁻⁵ Bq / mL according to the method described above. Example 3 Separation and quantitative analysis of ²¹⁰ Po Phosphoric acid solutions (85% by weight aqueous solution) for wet etching from various phosphoric acid manufacturers were prepared as sample solutions. 100 mL of each sample solution was added to a 300-mL quartz beaker, and 1 mL of a hydrochloric acid standard solution of Tracer ²⁰⁹ Po having a known concentration (1.0 × 10 ⁻² Bq / mL) was added. The tracer solution used here was manufactured by Oak Ridge National Laboratory as in Example 1, and had a T of ²⁰⁹ Po.
_1/2 = 103y. The sample solution after the addition of the tracer solution was sufficiently stirred and then left overnight. The next day, the sample solution was put into a thermostat together with the quartz beaker and heated to about 150 ° C. Next, the silicon wafer whose surface was washed with hydrofluoric acid (HF) was immersed in the heated sample solution, and heating was continued for 120 minutes.

After the completion of the heating, the silicon wafer was taken out of the sample solution and dried. Using this wafer as a radiation measurement sample, polonium (Po) deposited on the surface thereof was quantified by α-ray spectrometry in the same manner as in Example 2. As in the case of the above Example 2, ²¹⁰ Po is 10 ⁻⁵ Bq /
It was found that quantitative analysis can be performed with high accuracy and high reliability down to the level of mL. Example 4 Separation and Quantitative Analysis of ²¹⁰ Pb Phosphoric acid solutions for wet etching from various phosphoric acid manufacturers (all 85 wt% aqueous solutions) were prepared as sample solutions. To 5 mL of each of these sample solutions, 1 mL of a hydrochloric acid standard solution of tracer ²¹² Pb (thorium series) having a known concentration (0.01 Bq / mL) was added. In the tracer solution used here, T _1/2 of ²¹² Pb was 10.6 h. After allowing the sample solution after the addition of the tracer solution to stand overnight, methyl red was added as a pH indicator, and an appropriate amount of an aqueous sodium hydroxide solution was added dropwise to make the solution weakly acidic (pH = about 2).

Subsequently, the sample solution after pH adjustment was
Inject into the same electrodeposition cell (see Figure 2) as used in 1
Was. Electrodeposition was performed under the same conditions as described in Example 1 above.
The Pb contained in the sample solution was replaced with a stainless steel electrode plate.
Was deposited on the surface. After the electrodeposition process is completed,
The stainless steel electrode plate was taken out and dried. Then the electrodes
Regarding Pb deposited as a thin film on the surface of the plate,²¹²P
b (239 KeV) and²¹⁰By Pb (47 KeV)
Each gamma ray was measured. Commercially available for this gamma ray measurement
Tracer measurement device, ORTEC γ-ray measurement device
used. Then, based on the result of this γ-ray measurement,
²¹²Pb and²¹⁰Each count number and addition amount of Pb
From²¹⁰Pb was quantified. For each sample solution
²¹⁰From the results of Pb quantification, according to the method described above,²¹⁰
Quantitative analysis of Pb with high accuracy and high reliability
There was found.Example 5 ²¹⁰ Separation and quantitative analysis of Bi Phosphoric acid solution for wet etching of various phosphoric acid manufacturers
Liquids (all 85% by weight aqueous solution) are prepared as sample solutions
did. 0.01 B for each 10 mL of these sample solutions
q / mL tracer²⁰⁷Bi (T_1/2= 8.04y) in 1 mL
Was added.

Next, a solution of dithizone in carbon tetrachloride was added to the sample solution after the addition of the tracer to extract Bi into the organic phase. The organic phase was separated and combined with a solution of 2,5-diphenyloxazole in xylene. In the resulting mixture
²¹⁰ Bi was measured with a liquid scintillation counter.
Further, in order to measure ²⁰⁷ Bi in this solution, 0.5697 MeV γ-ray was measured using a germanium detector. Next, the result of the γ-ray measurement, ²⁰⁷ Bi and
^From the respective count numbers and the added amount of ²¹⁰ Bi, ²¹⁰ Bi
Was quantified. From the results of ²¹⁰ Bi quantification in each sample solution, it was found that ²¹⁰ Bi can also be quantitatively analyzed with high accuracy and high reliability by this method. Example 6 Preparation of high purity phosphoric acid 100 mL of a commercially available phosphoric acid solution (85 wt% aqueous solution)
Heat to 50 ° C. and then 5 g of granular silicon (purity 99.
9999999%, manufactured by Wako Pure Chemical Industries, Ltd.
Heating was continued at 50 ° C. for 2 hours. Next, the granular silicon was separated from the phosphoric acid solution, and the content of the radioisotope (eg, ²¹⁰ Po) in the granular silicon was quantified by α-ray spectrometry. As a result, it was found that the content of the radioisotope in the granular silicon was significantly increased as compared with the content before the addition in the phosphoric acid solution. In other words, it has been found that the phosphoric acid solution after removing the particulate silicon contains only a trace amount of radioisotope.

According to the findings of the present inventors, although the phosphoric acid solution was heated to about 150 ° C. in the above example, the heating temperature is usually set to LOC in the semiconductor manufacturing process.
Satisfactory impurity isolation effects can be obtained at a temperature during wet etching (used as a mask of a silicon oxide film) of a silicon nitride film used in the OS method, generally about 110 ° C. to about 200 ° C. . Example 7 Manufacture of Semiconductor Device Two types of commercially available phosphoric acid solutions for wet etching (85% by weight) were analyzed in accordance with the method described in Example 1 above, and the concentrations of radioactive substances ( ²¹⁰ Po) in the solutions were 10 ^-3 each.
It was confirmed to be Bq / mL and 10 ^-4 Bq / mL. Next, an evaluation was made on the influence of the phosphoric acid solution on the occurrence of soft errors in a semiconductor device obtained when the phosphoric acid solution was used as an etching agent in the manufacture of a semiconductor device.
For this evaluation test, Satoh et al., "CMOS-SRAM Soft
-Error Simulation System ", Annu. Proc. Reliab. Ph
ys., 3nd, pp 339-343 (1994). Simulation conditions are 256-kbit CMOS-SRA
M, drive voltage = 4.0 V.

A silicon wafer is immersed in the above phosphating solution.
After immersion and wet etching,
To²¹⁰The concentration of Po was measured. Of phosphoric acid solution²¹⁰Po concentration
Is 10^-3 When Bq / mL, the wafer surface²¹⁰Po concentration
Is 3 × 10^-Four Bq / cm^TwoMet. Also, the phosphoric acid solution²¹⁰
Po concentration of 10^-Four When Bq / mL, the wafer surface^{Two Ten}
Po concentration is 3 × 10^-Five Bq / cm^TwoMet. These numbers
Is compared with the simulation values of Satoh et al.
Then [see FIG. 13 on page 343], the phosphoric acid solution
of²¹⁰Po concentration of 10^-3 When Bq / mL, wafer surface
Concentration 3 × 10^-FourBq / cm^TwoThen 5 × 10^Twofit
Of phosphoric acid solution²¹⁰Po concentration of 10^-Four When Bq / mL,
Wafer surface concentration 3 × 10^-Five Bq / cm^TwoThen 5 × 10¹fi
t. Where 5 × 10^TwoThe number fit is
Level that is acceptable as
10¹The fit value indicates that the soft error rate is small enough.
Value.Example 8 Manufacturing of Semiconductor Device The method described in Example 7 was repeated. However, in this example
Is a commercially available phosphoric acid solution for wet etching (85 wt.
%,²¹⁰Po concentration = 10^-3 Bq / mL) as described in Example 6 above.
Purification according to the method of (5 hours heating at about 150 ℃
Changed) and used. Of the pure phosphoric acid solution used
²¹⁰Po concentration is 10^-Four It was Bq / mL. This phosphoric acid solution
Wet etching by immersing silicon wafer in
The surface of the wafer²¹⁰Po concentration is 3 × 10^-Five
 Bq / cm^TwoWas found. Therefore, this c
The wafer soft error rate is 5 × 10¹estimated to be fit
I could have. 5 × 10¹The fit value is
As described above, the soft error rate is a sufficiently small value.Example 9 Manufacturing of Semiconductor Device The method described in Example 8 was repeated. However, in this example
Is²¹⁰Po concentration = 10 ^-Four In Bq / mL high purity phosphoric acid solution
Before immersing silicon wafer, back side of silicon wafer
Of the silicon wafer to avoid adverse effects from
A silicon nitride film having a thickness of 500 nm was formed on the back surface. The above example
When wet etching was performed in the same manner as in step 8,
On the wafer surface²¹⁰Po concentration is 2.8 × 10^-Four Bq / cm^Twoso
It was recognized that [the silicon nitride film was not backed
Sometimes 3.0 × 10^-Four Bq / cm^Two]. This means that nitriding
When not backing the silicon film,
Attached to the back of the wafer²¹⁰Po is the wafer surface
Lining the silicon nitride film
When,²¹⁰Po is adsorbed on the wafer surface
This is because it does not go around. That is,
Back side of con-wafer (referred to as “non-phosphoric acid treated area”
Forming a silicon nitride film in advance on
Very effective in reducing the occurrence of soft errors
It is.

Similarly, satisfactory results could be obtained when a silicon oxide film was formed on the back surface of the silicon wafer instead of the silicon nitride film.

[0046]

As described above, according to the present invention, not only it is possible to easily prepare an analytical sample for quantitatively analyzing impurities in phosphoric acid with high sensitivity, but also it is possible to easily prepare an analytical sample.
Quantitative analysis of impurities itself can be performed with high sensitivity,
The impurity control of phosphoric acid can be advantageously performed. Further, according to the present invention, as a result of effectively separating impurities from phosphoric acid, it becomes possible to prepare high-purity phosphoric acid containing no impurities. In addition, since such high-purity phosphoric acid can be used as a processing liquid, a soft error (memory error) caused by an adverse effect of impurities in a semiconductor memory can be avoided, and a semiconductor device can be manufactured with high reliability. It becomes possible.

[Brief description of the drawings]

FIG. 1 is a sectional view showing in order a process of forming a silicon oxide film having a LOCOS structure according to a method of the present invention.

FIG. 2 is a schematic diagram showing a configuration of an electrodeposition cell used in the embodiment of the present invention.

FIG. 3. ²⁰⁹ Po by α-ray spectrometry and
FIG. 3 is an α-ray spectrum diagram of ²¹⁰ Po.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 electrodeposition cell 2 electrode plate (cathode) 3 anode 4 electrolyte 5 power supply 11 silicon substrate 12 silicon oxide film 13 silicon nitride film 15 silicon oxide film

Continuation of front page (56) References JP-A-7-237910 (JP, A) JP-A-5-206100 (JP, A) JP-A-7-333116 (JP, A) JP-A-6-249770 (JP) , A) JP-A-5-275405 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01N 1/10 G01N 27/26 G01N 31/00 G01T 1/167 G21H 5/02

Claims (19)

(57) [Claims] In preparing an analytical sample for quantitatively analyzing radioactive impurities contained in phosphoric acid, after heating the phosphoric acid, a semiconductor substrate is immersed in the heated phosphoric acid to cover the substrate. A method for preparing an analysis sample, which comprises separating and separating only an analysis impurity. 2. In preparing an analytical sample for quantitatively analyzing radioactive impurities contained in phosphoric acid, the phosphoric acid is changed to exhibit a weak acidity, and then the electrolytic deposition of the weakly acidic phosphoric acid is performed. A method for preparing an analysis sample, comprising: separating only an impurity to be analyzed on an electrode and separating the same. 3. The method according to claim 1, wherein the impurities to be analyzed are Pb, Bi and P.
The method for preparing an analytical sample according to claim 1, wherein the method is a radioisotope selected from the group consisting of: o. 4. When quantitatively analyzing radioactive impurities contained in phosphoric acid, after heating the phosphoric acid, a semiconductor substrate is immersed in the heated phosphoric acid to precipitate only the impurities to be analyzed on the substrate, And α emitted from the precipitate
Line, characterized by measuring the intensity of β-rays or γ-rays,
Method for analyzing impurities in phosphoric acid. 5. When quantitatively analyzing a radioactive impurity contained in phosphoric acid, the phosphoric acid is changed so as to exhibit a weak acidity, and then the impurity to be analyzed is deposited on an electrode by an electrolytic deposition method of the weakly acidic phosphoric acid. A method for analyzing impurities in phosphoric acid, comprising precipitating only phosphoric acid and measuring the intensity of α-rays, β-rays, or γ-rays emitted from the precipitates. 6. The method according to claim 1, wherein the impurities to be analyzed are Pb, Bi and P.
The method for analyzing impurities according to claim 4, wherein the method is a radioisotope selected from the group consisting of o. 7. Handling Examples tracer for adding ²¹⁰ Po quantitative analysis of identity position body isotopes ²¹⁰ Po when the radioisotope is ²¹⁰ Po as an object to be analyzed impurity, the radioisotope is If a ²¹⁰ Pb ²¹⁰
Pb handling isotopes was added a of identity position body as a tracer for ²¹⁰ Pb quantitative analysis or the radioisotope ^{210 210} of identity position body by adding isotope ²¹⁰ Bi in case a Bi Bi, The method for analyzing impurities according to claim 6, wherein the method is used as a tracer for quantitative analysis. 8. The method according to claim 6, wherein the impurity is analyzed by analyzing a daughter nuclide or a progeny of the radioisotope as the impurity to be analyzed. 9. In purifying phosphoric acid containing impurities to prepare high-purity phosphoric acid, the phosphoric acid to be purified is heated, and then the semiconductor substrate is immersed in the heated phosphoric acid to form only the impurities on the substrate. , And removing the semiconductor substrate from the heated phosphoric acid. 10. In purifying phosphoric acid containing impurities to prepare high-purity phosphoric acid, the phosphoric acid to be purified is changed to have a weak acidity, and then the electrode is formed by electrolytic deposition of the weakly acidic phosphoric acid. A method for preparing high-purity phosphoric acid, comprising depositing only the impurities on the top and removing the electrode from the weakly acidic phosphoric acid. 11. The method for preparing high-purity phosphoric acid according to claim 9, wherein the impurity is a radioisotope selected from the group consisting of Pb, Bi, and Po. 12. In processing a substrate to be processed to manufacture a semiconductor device, the content of impurities contained in at least one processing step included in the manufacturing process is selected from the group consisting of Pb, Bi, and Po. A method for manufacturing a semiconductor device, characterized in that phosphoric acid having a concentration of 10 ^-3 Bq / ml or less is used as a treatment liquid, as defined by the concentration of a selected radioisotope. 13. The method according to claim 12, wherein the phosphoric acid is an etchant for selectively removing a thin film formed on a substrate to be processed. 14. The phosphoric acid heats phosphoric acid which is to be used as a treatment liquid, and then immerses a semiconductor substrate in the heated phosphoric acid to precipitate the impurities on the substrate.
14. The method according to claim 12, wherein the semiconductor substrate is prepared by removing the semiconductor substrate from the heated phosphoric acid. 15. The phosphoric acid changes phosphoric acid which is to be used as a treatment liquid so as to exhibit weak acidity, and then deposits the impurities on an electrode by an electrolytic deposition method of the weakly acidic phosphoric acid. 14. The method of manufacturing a semiconductor device according to claim 12, wherein the electrode is prepared by removing the electrode from the weakly acidic phosphoric acid. 16. The phosphoric acid is prepared by adding silicon or a compound thereof to phosphoric acid to be used as a treatment liquid, causing the silicon or the compound to adsorb and remove the impurities. The method of manufacturing a semiconductor device according to claim 12, wherein: 17. The method according to claim 12, wherein the treatment using the phosphoric acid as a treatment liquid is performed in the presence of a protective film in a non-phosphoric acid treatment region of the substrate to be treated.
17. The method for manufacturing a semiconductor device according to any one of items 16. 18. The method according to claim 17, wherein the protective film is already formed on the substrate to be processed or newly formed prior to the phosphoric acid treatment. A method for manufacturing a semiconductor device. 19. The method according to claim 17, wherein the protective film is a thin film of a nitride or an oxide.